Cowen in "The Complacent Class" argues that the modern world is good for people who are motivated by strong personal interests, whether record collecting, hiking, cooking, or obsessing with "Game of Thrones." These people, called matchers or enthusiasts, are "not trying to come out ahead of everyone else; rather, they seek to have some of their niche preferences fulfilled for the sake of their own internally directed happiness." And they're finding it easier than ever thanks to technological and social progress.

The modern world isn't so good, meanwhile, for people who are motivated by beating others, Cowen says. These people, called strivers, are suffering in an age of unprecedented global competition, which makes it harder to excel, and discouraged by global perspective, which makes it harder for them to feel like they’re on top.

"[Millennials] are not actually indifferent or lazy or lacking in enthusiasm—quite the contrary—but more and more of their passions take forms other than those of the old climb-the-social-ladder variety," Cowen writes.

"Millennials might therefore appear to be lacking to the older generations who don't quite get the new terms of competition and satisfaction. In reality, the Millennials are doing pretty well with respect to the options the world has given them, and they are helping move that world toward more contentment and also less interest in grand projects or topping previous records of achievement."

There is something to be said for striving. The central argument of"The Complacent Class" is a critique of liberal boomers and yuppies who twiddled their thumbs (tended their gardens, read their books) while American economic growth slowed, segregation increased, and millions of people were left behind. If more Americans had stuck to the bold ambitions of the 50s and 60s, he suggests, we might be in a better place.

Still, Cowen says, the millennial generation is admirable: "They too are part of the complacent class, and they are also its finest product and its most committed ideological carriers."